1. Field of the Invention
The present invention is directed to increasing spatial resolution of Hartmann type wavefront sensors, more particularly to using a hard aperture to limit the wavefront area being sensed.
2. Description of Related Art
Hartmann wavefront sensing began as a technique for optical metrology, but the adoption of the technique by the adaptive optics community changed its primary usage. The adaptive optics community developed high photon efficiency high-speed sensors. Recently Hartmann sensors have been developed to address the needs of optical metrology again. Today most optical metrology is done with interferometry because of the availability of commercial systems and the coupled high dynamic range and high resolution, but improvements to the Hartmann sensor allow it to compete with interferometry for the optical metrology market. Shack-Hartmann wavefront sensor systems are being sold today for approximately one quarter the cost of commercial interferometers. These Shack-Hartmann wavefront sensors have sensitivity comparable to commercial interferometers and offer vibration insensitivity and typically higher dynamic range in a single compact package.
One major drawback of the Shack-Hartmann wavefront sensor is the inherently limited resolution due to the size of the lenses in the lens array. To see features smaller than a lens diameter, the wavefront had to be magnified before entering the sensor with a lens or set of lenses. The disadvantage of this technique is that the magnifying lenses then impose their own aberrations and the field of view of the wavefront sensor is reduced. Magnification of the wavefront reduces the sensor""s field of view but allows for higher resolution at the cost of the dynamic range.
Wavefront sensors are also used for metrology applications, as set forth, for example, in U.S. Pat. No. 5,563,709 to Poultney. However, the sensor and the test object were not moved relative to one another, and the sensitivity is limited to the aperture of the sensor. Full object metrology when the object is larger than the aperture of the sensor is set forth in commonly assigned, co-pending application Ser. No. 09/340,502 entitled xe2x80x9cApparatus and Method for Evaluating a Target Larger than a Measuring Aperture of a Sensorxe2x80x9d filed Jul. 1, 1999, now U.S. Pat. No. 6,184,974, the entire contents of which are hereby incorporated by reference for all purposes. When the goal is to examine extremely small dimples or defects, e.g., 5 nm, having a small spatial extent, e.g., 1-2 mm, on the surface of a silicon wafer requires a wavefront sensor with sufficient resolution to measure the height, resulting in fairly long focal length lens arrays. However, long focal length lens arrays are also required to be fairly large in diameter. In other words, the diameter or aperture of the sensor needed to detect the extremely small height of the feature is now larger than the spatial extent of the feature, making the feature irresolvable by the sensor. Thus, while a sensitive enough sensor can be made, the spatial resolution has been compromised.
The present invention is therefore directed to a wavefront sensor that substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
It is an object of the present invention to create a sensor having both high spatial resolution and good height sensitivity. This combination high spatial resolution and good sensitivity may also enable a variety of other applications such as flat panel display measurement, roll and sheet glass, plastic or metal film measurement, and a variety of other uses.
More particularly, it is an object of the present invention to extract sub-lens features from the wavefront sensor, referred to herein as knife-edge wavefront sensing. The basic idea is to move the wavefront across the lens array in steps that are a fraction of a lens diameter. The hard edges of the lens aperture, like a knife-edge, adjust the section of the wavefront exposed to each lens. The tilt seen by each lens can be determined using centroiding for each of the different positions of the wavefront and the features of the wavefront smaller than a lens can be determined. Thus, in accordance with the present invention, the spatial resolution of the sensor is increased by using the hard aperture of the lens to limit the wavefront being sensed.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the present invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility without undue experimentation.